Exposure apparatuses which manufacture devices such as a semiconductor device have tackled an increase in productivity. In order to increase the productivity, there has recently been developed an exposure apparatus in which a plurality of substrate stages for supporting a substrate are arranged and exposure and other processes are simultaneously performed.
An example of a process other than exposure is a measurement process for focusing or alignment (positioning). In this case, a measurement optical system for focusing or alignment is arranged at a position apart from an exposure optical system.
Japanese Patent Laid-Open No. 2002-280283 discloses a technique of increasing the productivity by arranging two substrate stages for supporting a substrate and executing exposure and alignment information detection in parallel to each other for two substrates. In an exposure apparatus disclosed in patent reference 1, a projection optical system for exposing a substrate and an alignment optical system for detecting alignment information are arranged apart from each other, and two substrate stages can be swapped with each other under the projection optical system and alignment optical system. This reference discloses a method of measuring the positions of the substrate stages in the translation direction (direction perpendicular to the optical axis of the projection optical system) between the projection optical system and the alignment optical system.
In order to align the wafer surface to the focus position of a pattern at high precision in the above exposure apparatus, measurement of the position of the substrate stage along the optical axis must also be considered. The exposure position and processing position must be set apart from each other in the translation direction in order to avoid contact between the substrate stages. In this case, a position measurement means for the substrate stage along the optical axis must be separately adopted.
When, however, the position measurement means is separately arranged, measurement is interrupted while the substrate stage moves from the processing position to the exposure position, and the reference position must be set again after movement. As a method of setting the reference position, the substrate stage is abutted against the reference position in the Z direction, or the reference portion is detected by a detection optical system, but these methods delay the start of exposure. Detection of the reference portion by the detection optical system premises that the stage enters the detection range of the detection optical system along the optical axis, but the reference portion may fall outside the detection range depending on the precision of a guide which guides the stage. This poses a problem particularly when a displacement sensor such as an interferometer is used as the position measurement means.